Electronic systems, for example systems ranging from small personal computers to large central office telephone exchanges, characteristically generate and radiate electromagnetic interference. One of the sources of radiated electromagnetic interference is that portion of an electronic system which is characterized by a multitude of logic gates and bistable elements, often referred to as digital circuitry. During operation, the digital circuitry depends upon at least one source of clock signal pulses for an orderly timing of switching occurrences in the logic gates and bistable elements. An electromagnetic energy pulse accompanies each switching occurrence and propagates outwardly, away from the electronic system, directly through the air or space and also via any communications leads or power leads connected to the system. The effects of this energy upon other nearby electrical and electronic systems may vary greatly, however such effects are always deleterious. In extreme cases, electromagnetic interference has rendered an electronic system inoperative and in some rare instances an apparatus has been known to suffer sufficiently to be rendered inoperative even after the interference has been removed.
In view of the extremely serious consequences which may accompany electromagnetic interference, manufacturers have been forced, as a matter of practicality and at significant expense, to provide protection such as shielding of cabinets and cable runs, and filters for power and communications leads. The effect of this protection is to reduce both entry and egress of electromagnetic energy. One example of a cabinet design incorporating shielding is disclosed in U.S. Pat. No. 4,679,867 issued 14 July 1987 to S. Heldenbrand et al and titled "Cabinet for an Electronic Apparatus". Futhermore, in various countries national regulatory agencies have promulgated standards as to limits of permissable levels of electromagnetic radiation. Before an electronic apparatus may be sold or used, a supplier of the apparatus must have been able to demonstrate, to the satisfaction of the regulatory agency, that in normal operating circumstances the electronic apparatus does not radiate unwanted energy in designated frequency ranges in excess of the permissable levels. With the ever increasing densities of such electronic systems being brought into use in cities and towns, some of the regulatory agencies have been moved in recent years to further reduce the permissable levels of unwanted electromagnetic radiation, with the intent of stemming what might otherwise become a torrent of electromagnetic radiation pollution.
As is well known to persons of typical skill in the technology of digital circuit apparatus and systems, the generation of electromagnetic interference is a direct result of the clock synchronous manner in which these systems operate. Although a change of the conductive state of any one switch element is accompanied by only a minuscule amount of electromagnetic interference generation, millions of such devices all switching in step synchronism one with the other typically generate a substantial amount of potentially troublesome radiation. Recently larger digital apparatus such as main frame computers and telephone switching systems of old design and current manufacture have had to be redesigned or modified with retrofits in order to meet the more stringent requirements.